SiTime’s Silicon MEMS VCXO Surpasses Quartz
Posted By: Aaron Partridge

Last week SiTime introduced a new family of Voltage Controlled Oscillators (VCXOs) designed for telecom, networking and embedded applications. These oscillators, the SiT380X family, surpass quartz performance and provide new and important capabilities.

Voltage controlled oscillators are commonly used where frequency or phase must be matched to an external clock. The most common example is in data receivers. When data is transmitted, perhaps on a wire, an optical fiber, or a wireless link, it is encoded at a rate set by the transmitter's clock. The data must be parsed into the receiver at exactly the same rate and at the right phase, or it will be corrupted. To do this the receiver must match its clock to the transmitter’s clock.

The receiver compares clock information embedded in the signal against its own clock. It then fine tunes, or “pulls” its clock as needed.A typical receiver uses a quartz crystal to generate its clock. It pulls the clock by generating an analog voltage that it applies to varactor diodes that load its quartz crystal. Varying this voltage varies the capacitance on these diodes and tunes the frequency.

So far so good, but there are problems with these quartz VCXOs: One is that varactors are non-linear, so the oscillator's voltage to frequency function is non-linear and varies from part to part.

This makes it harder to build a good control circuit. Also, the varactors and quartz crystals have limited pull range. If a quartz oscillator were to be designed to pull more than a couple hundred parts per million (ppm) its performance would degrade dramatically. And sometimes a couple hundred ppm is not enough.

Worst of all, quartz VCXOs show frequency hops, pops, and jumps that are a function of the crystal's temperature and the oscillator's pull.These quick and sometimes retrograde frequency shifts often cause receivers to lose lock, which means that they stop following the transmitter's clock. Losing lock is severe and results in data loss and dropped connections.

None of these are problems for SiTime’s MEMS VCXOs.This is because we do not pull our resonators; instead we digitize the analog control voltages and adjust the output frequencies with fractional-PLLs. Our voltage to frequency functions are highly linear and do not vary from part to part. Our fractional-PLLs adjust the output frequencies exquisitely smoothly. Our frequency adjustment ranges can be set from very small for fine control to very wide for robust tracking. And our MEMS resonators have no activity dips so they have no hops, pops, or jumps.

What are the benefits? The SiT380X family of VCXOs offers sub-picosecond integrated jitter, sub-percent pulling linearity, control ranges configurable from ±25 ppm to an astounding ±1600 ppm, and output frequencies from 1 to 220 MHz. This combination is physically impossible for varactor-pulled quartz and is an example of modern technology surpassing limits that were once thought to be insurmountable.

Learn more about SiTime’s MEMS Oscillators


Feb 11, 2022

Jan 11, 2022